Method of sensitizing foundation members for electric discharge devices



W. LAUTSCH July 30, 1940.

METHOD OF SENSITIZING FOUNDATION MEMBERS FOR ELECTRIC DISCHARGE DEVICES Filed April 26, 1938 Patented July 30, 1940 UNITED STATES aztaszi METHOD OF SENSITIZING FOUNDATION IVIEM'BERS FOR ELECTRIU DISCHARGE DEVICES Werner Lautsch, Berlin-Reinickendorf, Germany,

assignor to General Electric Company, a corporation of New York Application April 26, 1938, Serial No. 204,404 In Germany May 15, 1937 2 Claims.

The present invention relates to an improved process for coating a foundation member, such as a plate of metal, oxidized or otherwise treated, with a homogeneous layer of sensitive material. Plates of this character have various uses inthe art depending upon the character of the sensitive material. tion member is coated with caesium oxide, the surface not only responds to' light but also profusely emits secondary electrons when struck by primary electrons traveling at considerable velocities. In order to coat a number of these plates simultaneously with the sensitive: material, it has been customary to mount the plates in an enclosed chamber and tocondense the sensitive material from a vapor onto the plates. However, it has been found that when the plates are arranged fairly close together, the directions from which the vapor was condensed were such that the outer edge of the plates became more heavily coated than the inner portions thereof because the vapor did not have easy access to the innermost spaces between the plates. A tube in which the cathode foundation member is not uniformly coated gives erratic performance, regardless of whether the cathode is employed as a photo-electric or a secondary emission element.

Accordingly, an object of the present invention is to provide an improved method of coating a foundation member with a layer of sensitive material and having uniform thickness; Other objects are to provide a method by which the layer is formed by condensed particles moving in random directions which intercept all parts of the surface of the foundation member. A still further object is to provide a coating method which is applicable to those cases where a number of foundation members or plates are arranged close to one another with constricted.- spaces therebetween and are to be coated simultaneously. These objects are carried out in brief by vaporizing the sensitive material in the presence of gas under pressure whereby the vapor particles are caused to collide with the gas molecules and are given a multitudinous number of directions through the space about the foundation members. The uniform diffusion of the vapor through the gas causes the vapor to condense on the foundation member as a layer of uniform thickness.

The invention will be better understood when reference is made to the following description and the accompanying drawing in which Fig. 1 shows apparatus for carrying out the improved For example, when the foundaprocess, and Fig. 2 is a diagram useful in explaining the invention.

Referring more particularly to Fig. 1, numeral easily and, as illustrated, takes the form of a disk with a hemispherical portion at the center,

facing the left-hand bulb i; To the left of the 1 diaphragm 6, there is arranged a sharp, pointed piece of metal 1 with the point positioned to fracture the diaphragm when the metal member is moved either by an external magnet or by gravity. The bulbs I are contained in an oven 8, the temperature of which can be regulated or controlled for the purposes which will be explained hereinafter.

The foundation members or plates 9 which are to be coated either with light-sensitive material or with material suitable for secondary emission are mounted in any suitable manner in the righthand bulb l and arranged preferably in a parallel, horizontal position. The plates may be introduced througha large opening (not shown) in the right-hand bulb which is thereafter closed by melting the glass and drawing down to the exhaust tubulation 5. The interior of both bulbs is then highly evacuated by a pump (not shown) connectedto the tubulations 3 and 5.. The tubulation 5-is thereaftersealed ofi andthe stopcock 4 moved to its closed position. In case the sensitive material to be deposited on the plates 9 is caesium, which is light-sensitive as well as a good secondary emitter, the plates are preferably coated with nickel or silver and then oxidized before being placed in position. The caesium is then introduced into the-left-hand bulb in any suitable manner, for example, by heating a capsule (not shown) in the bulb and containing a compound of caesium and a reducing agent, or by distilling the material as a vapor through the tubulation 3. The caesium is indicated by the reference character ill. The oven 8 is then heated to a temperature sufficient to vaporize the caesium but the latter is prevented from reaching the plates 9 by the closed diaphragm 6. The diaphragm may be fractured by moving the weight 1 against the same, which can be conveniently done by means of a magnet on the outside of the tube or by moving the entire apparatus to a vertical position and allowing the weight to fall by gravity. The caesium vapor will then enter into the second chamber which is also maintainted at an elevated temperature by the oven 8. When the oven 3 is cooled, the caesium vapor condenses on all of the surfaces within the right-hand bulb including the foundation members 9. However, it is apparent that the directions taken by the caesium particles during the condensation step would be such that very little if any of the vapor would condense at the innermost spaces between the plates 9, particularly if the latter were arranged close to one another. The vapor would condense as a heavy layer around the outer edge of the plates, also on the upper surface of the uppermost plate and the lower surface of the lowermost plate. The plates would therefore not have a homogeneous layer of sensitive material over their entire surface and. when placed in a photo-electric tube or employed as secondary emitters, would not respond with equal effect to the impinging light rays or electrons over their entire surface.

In accordance with the present invention and as an improvement over the process described up to this point, an inert gas, preferably helium, is employed within the bulb to produce an equal distribution of the vapor throughout the bulb including the spaces between the plates 9. While the pressure of the gas employed for this purpose can best be determined by experiment, depending .upon the nature of the gas and of the sensitive material, I find that a pressure of several millimeters in the case of helium is desirable when caesium is being deposited The pressure of the gas should preferably be such that the mean free path between molecules is less than any dimension of the right-hand bulb I so that the vapor particles in moving about the bulb during the condensation process will strike one or more gas molecules, and these elastic collisions will serve to diffuse the vapor uniformly throughout the bulb. Consequently, when the vapor is being condensed on the plates 9, the particles thereof move in countless directions, and these directions necessarily are such as to cause the particles to move freely through the constricted spaces between the plates 9 and to strike all parts of the surface of each plate}: The vapor therefore condenses as a uniform layer over the entire surface of each plate and does not tend to condense more rapidly at the outer edges of anyone plate or on any other restricted portion thereof.

The manner in which the gas under pressure serves to cause an even distribution of the vapor throughout any and all of the spaces contained within the bulb is clearly shown in the diagram of Fig. 2. The foundation members to be coated are indicated at 9, and the molecules of gas are typified by the small circles Hi. It will be noted that these circles are spaced apart considerably less than the diameter of the bulb or combined length of both bulbs to indicate that the mean free path of the gas molecules is less than the important dimensions of the bulb or bulbs. The dotted lines ll represent the-directions taken by to condense thereon.

the vapor particles moving through the tube during condensation and colliding with the gas molecules or with the envelope. Inasmuch as these collisions are of an elastic character, the particles enter the constricted space between the plates 9 with great facility and at many different angles and directions, causing the vapor to condense at these inner spaces quite as readily as at the outer spaces near the periphery of the plates. I

When the rate of condensation determined by the reduction in temperature of the oven 8 is so rapid as to exhaust the limited supply of caesium vapor before the surfaces are entirely coated, it may be desirable to provide a continually moving stream of vapor in the bulb. The vapor can be introduced through the tubulation 3, either separately or combined with the inert gas. It has been found that when the vapor of a sensitive material is condensed in the presence of gas under pressure, the deposited layer, even though evenly distributed over the entire surface, is of a rough character. This condition is advantageous from the standpoint of multiple reflection of light which strikes the surface when the member is used in a photo-electric tube because the light tends to be reflected several times across each depression formed in the light-sensitive surface. At

each reflection or impingement of the light ray on the surface, the light causes increased reaction at the'surface which results in an increased light-sensitive effect.

While I have described the invention more particularly in connection with foundation members of a plate form coated with light-sensitive material, it will be understood that the invention is not limited to these features but can be applied to foundation members of any shape or material and to any coating material which can be readily vaporized.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In the 'art of simultaneously sensitizing a plurality of foundation members, the steps of oxidizing said members, then introducing them into an enclosure, heatingsaid members, passing helium at a pressure of several millimeters admixed with caesium vapor through spaces about and between said members, then cooling said members to cause the vapor to condense from random directions onto the entire surface of said members.

2. In the art of simultaneously coating a plurality of foundation members with a sensitizing material, the steps which include assembling the members in juxtaposition, providing relatively constricted spaces between them, supplying vaporized sensitizing material in the vicinity of the assembled members and in admixture with an inert gas at a pressure of the order of several millimeters so as materially to shorten the mean free path of the vapor molecules, thereby to assure their effective penetration into the said constricted spaces, and cooling the foundation members sufficiently to cause the vaporized material WERNER LAUTSCH. 

